Refractory material of vitreous silica

ABSTRACT

The invention concerns a refractory material comprised essentially of vitreous or amorphous silica and presenting an improved resistance to corrosion by molten metals, principally aluminium. It contains at least 0.1% and no more than 10% by wt. of barium sulfate. It has a crystallized silica content of less than 10% and it is comprised of at least 75% vitreous or amorphous silica.

BACKGROUND OF THE INVENTION

Sintered vitreous silica is a refractory material obtained by sinteringamorphous silica powder that has physical and chemical characteristicsthat make its application of particular interest for numerous industrialuses, notably for the metallurgy of nonferrous metals such as aluminium,zinc, brass, etc.

In effect, this material has a very low coefficient of thermalconductivity (0.7 W/m.K at 700° C.). This characteristic offerssignificant advantages. When the refractory element is used for thetransport or transfer of a molten metal, the heat losses by conductionthrough the wall of the receptacle or the transfer tube are reduced tothe minimum possible. Thus, it is not necessary to overheat the metal(i.e., heat is to a temperature higher than its melting point) to aconsiderable degree. This results in energy savings.

On the other hand, vitreous silica has a very low coefficient of thermalexpansion (0.6×10-6 m/K) when the product contains no crystallinephases. This low thermal expansion gives it an excellent resistance tothermal shocks. This makes it possible in most applications to use or toinstall a refractory element of vitreous silica without having topreheat it beforehand to bring it to a temperature close to itsoperating temperature. This results in a time gain and also an energysaving. This low thermal expansion permits thermal cycling a largenumber of times without mechanically fatiguing the material. It alsoassures a dimensional stability of the product with respect totemperature.

Furthermore, vitreous silica offers a good chemical stability withrespect to almost all the materials of molten metals and alloys.

This is why, among other applications, this material is used innonferrous metallurgy. For example, low-pressure foundry installationsare known, in which a mold is fed with metal through its lower part viaa tube dipping into a vat filled with molten metal. The pressure of apulsating gas, e.g., nitrogen or other neutral gas, is applied to thesurface of the molten metal to cause it to rise into the mold. Vitreoussilica is perfectly suited for the material of the dipping tube.

However, vitreous silica is corroded under the action of a metal thathas a greater affinity relative to oxygen. This is the case, forexample, of aluminium brought to its melting point or a highertemperature. This metal reduces silica to form a deposit of alumina,leaving metallic silicon. After a period of time that is a function ofthe thickness of the refractory element, this corrosion ends in thecracking or rupture of the said element. This phenomenon is observed forall applications where the vitreous silica refractory is in contact withthe molten aluminium.

A refractory composition resistant to molten aluminium has also beendisclosed (U.S. Pat. No. 4,992,395). This composition includes 10-18 wt% ceramic fiber. By ceramic fiber it is meant amorphous ornon-crystalline fiber composed of 50% alumina and 50% silica. Such a twophase material is essentially an insulating material suitable for makingcoatings. Its insulating properties are obtained by the low packingdensity caused by the fibers during the shaping process. This leads to aporosity of at least 20% which results in the insulating properties ofthe material. However, such a material does not have enough mechanicalresistance to make structural pieces out of it, that is pieces having amechanical resistance by themselves, and not simply to form the coatingof another structure.

SUMMARY OF THE INVENTION

The precise object of the present invention is a material comprisedbasically of vitreous silica, notably for use in nonferrous metallurgy,particularly aluminium, which remedies this shortcoming.

This refractory material is characterized in that it contains at least0.1% and no more than 10% by wt. of barium sulfate.

The presence of barium sulfate substantially reduces the wettability ofmolten metals on this refractory. Furthermore, it is compatible with theshaping process of the refractory, which for example can be slip castingor injection molding: pouring in the form of slips utilizes thesuspension in water of vitreous silica powder finely ground in water.The additive should be insoluble in water. With regard to thermoplasticinjection molding, the additive should have an adapted granulometry soas not to affect the injectability properties.

The vitreous silica products, once shaped, are densified by sintering attemperatures above 1000° C. This densification corresponds to a decreasein viscosity of the vitreous silica. Barium sulfate has a goodtemperature stability.

In effect, barium sulfate is a stable, refractory compound compatiblewith the densification temperatures of vitreous silica and furthermoreit does not dissolve in water. It is widely available in the naturalstate in the form of barite and the various granulometric sizesavailable are compatible with the shaping processes envisioned.

The barium sulfate content depends on the contemplated application andthe forming process. The higher it is, the more the corrosion phenomenain aluminium are reduced. On the other hand, high barium sulfatecontents diminish the mechanical properties and the resistance tothermal shocks due to the increasing proportion of crystallized silicain the finished product.

The incorporation of barium sulfate in the fused silica product takesinto account the forming process used and modifies the densificationconditions of the products formed.

The refractory material preferably has a content of crystallized silicaless than 10%.

This low proportion of crystallized silica permits preservation of thegood properties of resistance to thermal shock of molten silicaproducts.

The refractory material is also preferably comprised of at least 75%vitreous of amorphous silica.

According to another embodiment, the refractory material has a porosityof at least 30% and at most 80%.

The invention also concerns an article which is characterized in that itis comprised of a refractory material as described above, as well as anarticle that has an exterior surface coated with this refractorymaterial over a thickness of more than 2 mm.

The invention concerns a manufacturing process for an articlecharacterized in that a slip is prepared, and a slip of barium sulfate,in that these two slips are mixed and that the mixture is cast into amold.

The invention concerns a process for manufacturing an article,characterized in that silica grains and barium sulfate grains are mixed,that a slip is prepared, that a slip of this mixture is prepared andthat the said slip is cast into a mold.

Finally, the invention concerns a process for manufacturing an article,characterized in that silica grains and barium sulfate grains are mixedwith organic binders to obtain a thermoplastic paste that is injectedinto a mold.

A foaming agent may further be added to increase the porosity.

DETAILED DESCRIPTION OF THE INVENTION

Other characteristics and advantages of the invention will appear inreading the examples given below purely for illustrative purposes.

EXAMPLE 1

A tube was produced for feeding a low-pressure casting mold foraluminium and its alloys. The new product was produced by mixing a slipof vitreous silica and a slip of barium sulfate. The two slips hadessentially the same characteristics of density, grain size andfluidity. The barium sulfate slip represented 5% of the total mixturecast. The mixture was poured into a plaster mold and then dried. The newproduct was fired at a temperature above 1050° C. The material had aporosity of 12%, a density of 1.93 g/cm³ and a MOR of 2000 PSI (cold MORin flexion). Results: the service life of the tube was multiplied atleast by three relative to that of a traditional tube of pure vitreoussilica produced by pouring a slip.

EXAMPLE 2

A pouring tube was produced, also for aluminium and its alloys. Thistube was produced of vitreous silica by an injection molding process.This process consists of mixing dry powders of a specific granulometrywith binders comprised of thermoplastic organic materials. Injection andthen cooling of the mold permit obtaining articles of a form that willbe subsequently debonded and then baked. The incorporation of a bariumsulfate powder with a granulometry identical to that of conventionalsilica powder made it possible to obtain tubes containing 5% by wt. ofbarium sulfate. The reduction in the melting point made it possible topreserve the mechanical properties of the products while limiting theportion of crystallized silica to less than 5%.

Results: the service life of the tube was multiplied by four relative tothat of a conventional tube of pure vitreous silica produced by pouringa slip.

EXAMPLE 3

A runner for transferring molten aluminium was produced by mixing a slipof vitreous silica and a slip of barium sulfate representing 5% of thetotal mixture cast. The mixture was poured into a plaster mold, driedand fired at a temperature of 1050° C. or higher.

The product had a porosity of 50%, a density of 1.1 g/cm³, and a MOR of700 PSI. The thermal conductivity was less than 0.5 W/m.K at 700° C. Theservice life was multiplied by two with respect to a similar runner madeof foamed silica.

We claim:
 1. A refractory material having an improved resistance tocorrosion by a molten metal that has an affinity relative to oxygengreater than that of silica, comprising vitreous silica and at least0.1% and no more than 10% by weight of barium sulfate.
 2. The refractorymaterial according to claim 1, including a crystallized silica contentof less than 10% by weight.
 3. The refractory material according toclaim 1, comprising at least 75% by weight vitreous silica.
 4. Therefractory material according to claim 1, having a porosity of between30% to 80%.
 5. The refractory material according to claim 1, wherein themolten metal is aluminium.
 6. An article for the transfer of moltenmetal having an affinity relative to oxygen greater than that of silica,said article having at least a molten metal contacting surface made froma refractory material comprising vitreous silica and between about 0.1%and 10% by weight barium sulfate.
 7. The article of claim 6, wherein themolten metal contacting refractory surface has a thickness of more than2 mm.